Process for making a foamed elastometric polymer

ABSTRACT

The present invention relates to a process for forming a foamed elastomeric polymer. The process involves forming a reverse emulsion of liquid droplets in a continuous liquid phase of polymer precursor and then polymerizing the precursor to entrap uniformly distributed droplets of the liquid in pores formed in the polymer bulk. The liquid in the pores is then removed under supercritical conditions.

FIELD OF THE INVENTION

The present invention relates to a process for making a foamedelastomeric polymer for use as a dielectric in electronic components.

Background of the Invention

There is a continuing desire in the electronics industry to increase thecircuit density in electronic components, e.g., circuit boards,multichip module chip test devices, and the like without degradingelectrical performance (e.g., crosstalk) and also to increase the speedof signal propagation in these components. One method of accomplishingthese goals is to reduce the dielectric constant of the polymericinsulator used in the components. A method for reducing the dielectricconstant of a polymeric insulator is to foam the polymer with verysmall, uniformly dispersed air-filled pores.

Polymeric foams are well known in the art. One prior art process ofmaking polymeric foams involves dispersing thermally decomposableparticles in the monomer reaction mixture. The mixture is then heated tocause simultaneous polymerization and thermal decomposition of theparticles to form a gas. The gas expands to form pores in the polymerresin. Unfortunately, the process results in nonuniform pore sizes andnonuniform distribution of pores throughout the bulk.

Another process was disclosed by Even et al., in the Material ResearchSociety Bulletin, April 1994. Even discloses an emulsion process forforming foams of polystyrenes. The process involves forming an emulsionof water droplets in a continuous styrene monomer phase and thenpolymerizing the monomer to form a continuous bulk of thermoset polymerwith dispersed water droplets. The water is then removed by heating toform the foam. Unfortunately, the Even process is not suitable forforming elastomeric polymeric foams because during the heating step, thepores collapse with the removal of the water which draws the elasticwalls of the pores permanently closed.

It is therefore an object of the present invention to provide animproved process for making a foamed elastomeric polymer suitable foruse in electronic devices.

Other objects and advantages will become apparent from the followingdisclosure.

SUMMARY OF THE INVENTION

The present invention relates to a process for forming a foamed polymer.The process involves (a) forming a reverse emulsion of liquid dropletsin a continuous liquid phase of polymer precursor; (b) polymerizing theprecursor to entrap uniformly distributed droplets of the liquid whichform the corresponding pores in the polymer bulk; (c) heating thepolymer under pressure in the presence of a gas to conditions near orabove the critical temperature and critical pressure of the liquid inthe pores to thereby exchange the liquid entrapped in the pores with thegas. Optionally, the polymer of step (b) can be first contacted with anorganic solvent to exchange the liquid in the polymer pores with anorganic solvent. Preferably the process relates to forming foamedelastomeric polymers such as polysiloxane. The foamed polymer made bythe process of the invention has uniformly dispersed pores havinguniform pore sizes with pore sizes preferably ranging from about 1 to 8microns.

A more thorough disclosure of the present invention is presented in thedetailed description.

Detailed Description of the Invention

The present invention relates to a process for forming a foamed polymer.Preferably, the process involves four steps.

The first step involves forming a reverse emulsion of a discontinuousphase of liquid droplets in a continuous liquid phase of polymerprecursor. Suitable polymer precursors include monomers and oligomers. Avariety of foamed polymers can be made by the process of the presentinvention. Suitable polymers formed in the process of the presentinvention include polyurethane, polysiloxane, polybutadiene andpolymethacrylate.

A preferred elastomeric polymer is polysiloxane (e.g.,polydimethyl-codiphenyl siloxane) such as disclosed in U.S. Pat. No.5,441,690, the disclosure of which is incorporated herein by reference.Suitable precursors for polysiloxane are siloxane monomers or oligomerswith reactive end groups. Suitable reactive end groups are vinyl,hydroxy, alkynyl and acetoxy.

Suitable liquids for forming the liquid droplets in the emulsion includepolar liquids such as water, C₁₋₆ alcohols (e.g., ethanol, isopropanol)and C₃₋₆ ketones (e.g., acetone). Other suitable liquids will be knownto those skilled in the art.

Preferably, the emulsion mixture contains a surfactant to reduceinterfacial tension. A suitable surfactant for forming polysiloxanefoams is the block copolymer of ethylene oxide-dimethylsiloxane. Othersuitable surfactants include ionic and nonionic surfactants.

Other polymeric additive may also be added to the emulsion mixture suchas fillers (e.g., inorganic fillers such as zinc oxide).

The emulsion can be formed by art known methods such as mixing thecomponents in a high speed blender or mixer or ultrasonic mixer. Theemulsion is stable and injection moldable.

The second step involves polymerizing the precursors. Preferredprecursors for polysiloxane are vinyl terminated siloxane oligomers. Forpolysiloxane foams, polymerization is suitably initiated byhydrosilation with platinum catalyst at an elevated temperature.Polymerization results in the formation of crosslinked polysiloxane. Thediscontinuous phase of tiny droplets of the polar liquid forms uniformlydispersed pores throughout the polymer bulk having uniform pore size.

The third step of the preferred process of the present inventioninvolves contacting the polymer bulk formed in step two with an organicsolvent to exchange the entrapped polar liquid with the organic solvent.Suitable organic solvents include C₁₋₆ alcohols (methanol, ethanol,isopropanol), C₃₋₆ ketones (acetone) and C₅₋₁₀ alkanes (hexane).Suitable organic solvents will suitably be miscible with the liquidalready in the pores and nonswelling in the polymer matrix. Thepreferred organic solvent is acetone. Conveniently, the polymer bulk issoaked in the organic solvent for a period of time of e.g., 2 hours in asohxlet extractor.

The last step of the process of the present invention involves heatingthe polymer bulk under pressure with a gas to a temperature and pressurenear or above the critical temperature and critical pressure of thefluid in the pores. Under these atmospheric conditions, the fluid in thepores becomes a critical fluid. A, critical fluid, as used herein, is asubstance heated to a temperature near or above its critical temperatureT_(c) and compressed to a pressure near or above its critical pressureP_(c) to achieve miscibility with no phase separation. As used herein, atemperature near or above the T_(c) will be a temperature greater thanabout 15° C. below the T_(c). As used herein, a pressure near or aboveP_(c) will be a pressure greater than about 1 atmosphere below theP_(c). Preferably, the critical fluid is at or above the T_(c) and at orabove the P_(c) e.g., a supercritical fluid. At its critical temperatureand pressure, the organic fluid can be removed from the polymer bulkwithout causing collapse of the pores formed in the polymer. Step fourcan be accomplished by placing the bulk polymer in an enclosed pressurevessel with a gas and heating the vessel under pressure to conditionsnear or above the T_(c) and P_(c) of the organic liquid in the pores.

Suitable gases include air, nitrogen and argon. After the gas hasexchanged out the critical fluid in the pores, heating is discontinuedand the temperature and pressure are slowly lowered to ambient.

The foamed polymer formed by the process of the present invention hasuniformly distributed pores throughout the bulk of the polymerpreferably on the order of 1-2 micron. Further, the pore size can beadjusted (from submicron to about 30 microns) by varying the surfactant,stirring speed and cure rate. The polymer has low stress, low dielectricconstant, improved toughness and improved compliance during mechanicalcontacting to require less contact force during compression. The polymermade by the process of the present invention is suitable for use inelectronic devices including an integrated circuit chip test probe andpinless connector for packaging.

The following examples are a detailed description of the process of theinvention. The detailed description falls within the scope of, andserves to exemplify, the more generally described process set forthabove. The examples are presented for illustrative purposes only, andare not intended as a restriction on the scope of the invention.

EXAMPLE 1

1. Emulsion Components

--polymer precursor--Dow Corning Sylgard 182 (siloxane oligomer withcuring agent)

--surfactant--dimethylsiloxane ethylene oxide block copolymer (ES224 bySilicon Resources Inc.)

--catalyst--dicyclopentadiene--PtCl₂ complex

Several emulsions were obtained by mixing polymer precursor, water, thesurfactant, the curing agent and the catalyst, in the given order. Theamount of surfactant was varied in each emulsion from 5 to 20% of watermass. The mass ratio of precursor to curing agent was 10 to 1. The twophase system was stirred at high speed with an overhead stirrer equippedwith a fan shaped stirrer blade. The homogeneous emulsion was achievedafter about 5 minutes of stirring and then degassed in a vacuum ovenuntil no more bubbles were observed. The samples were oven cured atapproximately 70° C. for about 4 hours, in sealed disposable flasks. Thewater was then exchanged with acetone using Sohxlet extractor. Theextraction time was one hour at a temperature of 85° C.

The samples were placed in a pressure chamber whose pressure was raisedto 60 atm. The temperature was raised to 250° C. at a rate of 5° C./minand at constant pressure. The pressure was then reduced at a constanttemperature at a rate of 2 atm/min. On reaching the ambient pressure,the temperature was reduced at a rate of 5° C./min. At 200° C. thesystem was purged with air, and allowed to continue cooling to ambientto form the foamed polysiloxane product. The mean pore sizes of the foampolysiloxane samples varied from 200 nm to 5 microns, depending on theamount of surfactant used.

Although this invention has been described with respect to specificembodiments, the details thereof are not to be construed as limitationsfor it will be apparent that various embodiments, changes, andmodifications may be resorted to without departing from the spirit andscope thereof, and it is understood that such equivalent embodiments areintended to be included within the scope of this invention.

We claim:
 1. A process for forming a foamed polysiloxane comprising thesteps of:(a) forming an emulsion of liquid droplets dispersed in acontinuous liquid phase of polysiloxane precursor; (b) polymerizing thepolysiloxane precursor to form a polysiloxane having dispersed poresfilled with the liquid; and (c) heating the polysiloxane under pressurewith a gas to a temperature above about 15° C. below the criticaltemperature of the liquid and a pressure above about 1 atmosphere belowthe critical pressure of the liquid to exchange the liquid in the poreswith the gas.
 2. The process of claim 1 wherein the temperature is at orabove the critical temperature of the liquid and the pressure is at orabove the critical pressure of the liquid.
 3. The process of claim 2wherein the liquid is an alcohol or ketone.
 4. A process for forming afoamed polysiloxane comprising the steps of:(a) forming an emulsion ofpolar liquid droplets dispersed in a continuous liquid phase ofpolysiloxane precursor; (b) polymerizing the polysiloxane precursor toform a polysiloxane having dispersed pores filled with the polar liquid;(c) contacting the polysiloxane with an organic liquid to exchange thepolar liquid in the pores with organic liquid; and (d) heating thepolysiloxane under pressure with a gas to a temperature above about 15°C. below the critical temperature of the organic liquid and a pressureabove about 1 atmosphere below the critical pressure of the organicliquid to exchange the organic liquid in the pores with the gas.
 5. Theprocess of claim 4 wherein the temperature is at or above the criticaltemperature of the organic liquid and the pressure is at or above thecritical pressure of the organic liquid.
 6. The process of claim 5wherein the organic liquid is a ketone.
 7. The process of claim 6wherein the organic liquid is acetone.
 8. The process of claim 7 whereinthe foamed polymer has pore size of about 1 to about 8 microns.